The present invention relates to a photo-electric detector particularly for the conversion of infrared radiation into an electrical signal. More particularly, the invention relates to an infrared detector which is simple to make, has extremely high geometric resolution or resolving power, very high contrast range, and a low leakage or dark current. Such a detector works usually in accordance with photo-electric field emission and is advantageous in many instances and provides particularly new fields of employment and alternative for photo cells, photo multipliers, image converters, and electron beam image tubes, such as vidicons.
Night sight viewing systems available at the present time can be placed into two categories. In accordance with the first category, residual or remaining ambient light is amplified. Devices of this kind are based on the principle of using photo-emitting surface layers, such as photo cathodes having very high sensitivity with regard to visible radiation as well as in the near infrared range covering wave lengths up to about 900 nanometers. Systems of this kind are relatively inexpensive, compact, and yield quite an adequate amount of power. However, their effectiveness is strongly interfered with by haze and fog.
The second category of so-called thermal image devices operate in frequency range from 3 to 13 micrometers, and respond particularly to the inherent radiation emanating from any and all objects on account of their temperature. Generally, this phenomenon is known as thermal radiation. Systems of this kind have an enhanced detection range and are operative also under fairly bad weather conditions. However, they require rather extensive and expensive image conversion and processing techniques. Examples here are vidicon image tubes, optical mechanical scanning, and self-scanning integrated circuits. Moreover, the sensor field has to be cooled in case it is used in connection with semi-conductors.
Generally speaking it has always been the practice to shift the operation limit lamda-K of photo cathodes towards longer wave lengths. An electron emitting photo detector operating actually in the range from 1 to 2 micrometers and above would offer the following advantages:
The power output, particularly for night sight and night vision devices of the first-mentioned category would be increased, i.e., the residual light amplification would be enhanced. The photon influx of the night sky as well as the reflection of chlorophyll in plants increase drastically for wavelengths above 900 nanometers so that one can obtain images with a significant higher contrast.
The intensity of the radiation of a cloud-covered night sky at about a wave length of 1.6 micrometer is about the same as the thermal radiation of objects under normal ambient temperatures. This means that a detector which is particularly sensitive in the one to two micrometer band is not only useful as a residual light amplifier, but also as a thermal detector. Since both methods operate complimentary as far as object properties is concerned, it can readily be seen that the coupling of these two methods produces a higher information content, and a higher sensitivity so that completely new and not yet really foreseeable possibilities and feasibilities of night-sight engineering may obtain.
Detecting and acquiring radiation in the temperature radiation range by means of photo cathodes, particularly for wave length in excess of 2 micrometers, would actually entail a drastic simplification and lowering of cost of the cameras because the advantages of the two categories mentioned above can in fact be combined in a mutually reinforcing fashion so that one may even speak of synergism. The simplification is particularly very the result of the fact that released photo-electrons can be used immediately and directly for the production of an image on a video screen, such as is the practice, for example, in so-called cascade tubes or in multi-micro channel tubes.
Thus far the advantages offered above are merely the result of a hypothetical situation and of speculation concerning the realization of an extension of the effective wave length range. In the past, certain proposals have been made for extending the detection range of photo cathodes to above 1 micrometer. The problem should be considered in some detail. In the case of photo-electron emission, the incoming photons must have a quantum energy hv.sub.k which is larger than or equal to the electron work function in order the photo effect, i.e., the work function is a physical constant for the particular material and is therefore decisive for the limit frequency v.sub.k, and the limit wave length lamda.sub.k which is equal to c/v.sub.k wherein c is the velocity of light. In the case of semi-conductors the situation is somewhat modified because the fermi level is not occupied. In the case of a P-type semi-conductor in which only the valence band is filed with electrons, a photo-electron must have a minimum energy which is the sum of the band spacing and of the electron affinity, otherwise emission is not possible. The work function of most metals is about 4.5 electron volts, which corresponds to a limit wave length lamda k of about of about 0.28 micrometers. That of course means that ultra-violet light is necessary in order to release photo electrons.
The known photo cathodes have a work function which has been lowered through suitable surface treatment, such as coating the electrode carrier with cesium or a cesium compound. This way one may be able to reduce the work function to about 1 electron volts. Accordingly, such a photo cathode is sensitive, not only to visible light, but also to infrared radiation of frequencies below the visible light spectrum. Further reduction of the work function has been carried out primarily through two methods: These are called the NEA method and the field assisted photo emission.
In the case of the NEA method (negative electron affinity, Journal of Electron-Materials Vol. 3, No. 9, 1974 by means of strong bending of the band edges due succeeds that the vacuum level E.sub.vac is lowered below the conduction band edges E.sub.L. The electron affinity is the difference between these two values, i.e. (E.sub.a =E.sub.vac -E.sub.L) and this difference is rendered negative so that the electrons which were to occupy the conduction band can in fact leave the solid. Even though such NEA cathodes are known for 20 years, and even though laboratory tests produced quite a good quantum yield, the realization of this concept leaves much to be desired, particularly for reasons of manufacturing which proved to be more difficult than was anticipated. Moreover, the NEA cathodes have a principal drawback on account of the particular semi-conductor used, such as gallium-arsenide and silicon because in view of the distance between the conduction and valence bands one obtains a limit of the spectrum region, i.e., the absorption edge is near 1.12 micrometer for silicon and 0.92 micrometer for gallium-arsenide.
In the so-called field assisted photo emissions one uses the known electric point effect. This effect is characterized that a high electric field strength occurs on a sharp point or edge, and this has the effect that the potential barrier at the surface of the solid is lowerer and reduced as to width. Without an electric field, there is a step-shaped dependency of the potential energy of an electron from the distance from the metal surface, but in the presence of a strong electric field this steplike dependency is modified to a lower wall or barrier. Due to the tunnel effect, electrons may leave the solid even if the energy is smaller than the work function. In the case of metal, electrons will be emitted from states directly below the fermi level. The field electron microscope is a known, practical realization of this effect.
Photo field cathodes (PFE cathodes)) are realized in test cases under utilization of semi-conductor materials, as was reported, for example, in, IEEE Transaction on Electronic Devices Vol. 21, page 785, 1974. For this purpose one provides a plurality of points or peaks on a semi-conductor crystal made of silicon under utilization of a locally selective etching. A typical radius r of curvature of these peaks is about 100 .ANG.ngstrom. The distance between the peaks is in the vicinity of two times their height h and equals 20 micrometers. A typical and somewhat schematical example of this type is shown in FIG. 1. The ratio of the emitting surface towards the total surface is given by r.sup.2 /h.sup.2, and is in this case about 2.5.10.sup.-8. In order to obtain a high sensitivity, it is desirable that not only photo electrons are produced in the immediate vicinity of the peaks and leave the surface, but a large area of the crystal should contribute to the photo current. This, however, is possible only if two rather elementary premises are fulfilled:
The penetration depth of the light must be large as compared with the height h of the peaks. Moreover, the extension and range of the photo electrons within the solid, i.e., their diffusion length, must be approximately equal to this penetration depth because electrons produced within the material are at least supposed to reach the surface. It was found that on the basis of weakly doped P-type silicon material favorable values for the penetration depth as well as for the diffused length, in the order of hundred micrometers can be obtained. One can see from this example moreover, the total photo field emission is presently realizable only on the basis of semi-transparent semi-conductor material. The penetration depth of light in metal is however much more limited, particularly in view of the high electron density, this penetration depth is usually not more than 1/10th of the respective wave length so that metal structures of the type shown in FIG. 1 will in fact be photo sensitive only in the immediate vicinity of the peaks, and this in turn means that the detector sensitivity measured in relation to the total surface is very small indeed.
The known semi-conductor photo field emitters are encumbered by a number of rather serious disadvantages which in fact may render questionable any such successful employment for infrared cathodic photo detection. The finite band spacing of the semi-conductor results in a limit wave length which is in fact quite similar to those of the NEA cathodes, and even of the conventional photo cathodes. The activation of photo electrons in the entire surface area between the peaks which is absolutely necessary in order to obtain a sufficiently high quantum yield has undesirable and even unavoidable side effects: One produces a very high dark current on account of the thermal excitation of surface states which are always present. In view of the large diffusion lengths, and in further view of the field effect, the thermally produced electrons will reach the peak area with a probability of nearly 100% and thus simulate the effect produced by photo electrons. These thermally produced electrons will therefore be emitted analogously. This means that a satisfactory operation of such semi-conductor cathode is possible only if it is very strongly cooled in order to eliminate this parasitic thermal effect.